Oil and gas well production suffers from basic mechanical problems that may be costly, or even prohibitive, to correct, repair, or mitigate. Friction is ubiquitous in the oilfield, devices that are in moving contact wear and lose their zo original dimensions, devices may be degraded by corrosion and erosion, and deposits on devices can stick and impede their operation. These are all potential impediments to successful operations, and all five mechanical problems, friction, wear, corrosion, erosion, and deposits, may be mitigated by selective use of coatings as described below.
Drilling Rig Equipment:
Following the identification of a specific location as a prospective hydrocarbon area, production operations commence with the mobilization and operation of a drilling rig. In rotary drilling operations, a drill bit is attached to the end of a bottom hole assembly, which is attached to a drill string comprising drill pipe and tool joints. The drill string may be rotated at the surface by a rotary table or top drive unit, and the weight of the drill string and bottom hole assembly causes the rotating bit to bore a hole in the earth. As the operation progresses, new sections of drill pipe are added to the drill string to increase its overall length. Periodically during the drilling operation, the open borehole is cased to stabilize the walls, and the drilling operation is resumed. As a result, the drill string usually operates both in the open borehole (“open-hole”) and within the casing which has been installed in the borehole (“cased-hole”). Alternatively, coiled tubing may replace drill string in the drilling assembly. The combination of a drill string and bottom hole assembly or coiled tubing and bottom hole assembly is referred to herein as a drill stem assembly. Rotation of the drill string provides power through the drill string and bottom hole assembly to the bit. In coiled tubing drilling, power is delivered to the bit by the drilling fluid. The amount of power which can be transmitted by rotation is limited to the maximum torque a drill string or coiled tubing can sustain.
In an alternative and unusual drilling method, the casing itself is used to drill into the earth formations. Cutting elements are affixed to the bottom end of the casing, and the casing may be rotated to turn the cutting elements. In the discussion that follows, reference to the drill stem assembly will include a “drilling casing string” that is used to drill the earth formations in this “casing-while-drilling” method.
During the drilling of a borehole through underground formations, the drill stem assembly undergoes considerable sliding contact with both the steel casing and rock formations. This sliding contact results primarily from the rotational and axial movements of the drill stem assembly in the borehole. Friction between the moving surface of the drill stem assembly and the stationary surfaces of the casing and formation creates considerable drag on the drill stem and results in excessive torque and drag during drilling operations. The problem caused by friction is inherent in any drilling operation, but it is especially troublesome in directionally drilled wells or extended reach drilling (ERD) wells. Directional drilling or ERD is the intentional deviation of a wellbore from the vertical. In some cases the inclination (angle from the vertical) may be as great as ninety degrees. Such wells are commonly referred to as horizontal wells and may be drilled to a considerable depth and considerable distance from the drilling platform.
In all drilling operations, the drill stem assembly has a tendency to rest against the side of the borehole or the well casing, but this tendency is much greater in directionally drilled wells because of the effect of gravity. The drill stem may also locally rest against the borehole wall or casing in areas where the local curvature of the borehole wall or casing is high. As the drill string increases in length or degree of vertical deflection, the amount of friction created by the rotating drill stem assembly also increases. Areas of increased local curvature may increase the amount of friction generated by the rotating drill stem assembly. To overcome this increase in friction, additional power is required to rotate the drill stem assembly. In some cases, the friction between the drill stem assembly and the casing wall or borehole exceeds the maximum torque that can be tolerated by the drill stem assembly and/or maximum torque capacity of the drill rig and drilling operations must cease. Consequently, the depth to which wells can be drilled using available directional drilling equipment and techniques is ultimately limited by friction.
One string of pipe in sliding contact motion relative to an outer pipe, or more generally, an inner cylinder moving within an outer cylinder, is a common geometric configuration in several of these operations. One prior art method for reducing the friction caused by the sliding contact between strings of pipe is to improve the lubricity of the annular fluid. In industry operations, attempts have been made to reduce friction through, mainly, using water and/or oil based mud solutions containing various types of expensive and often environmentally unfriendly additives. For many of these additives the increased lubricity gained from these additives decreases as the temperature of the borehole increases. Diesel and other mineral oils are also often used as lubricants, but there may be problems with the disposal of the mud, and these fluids also lose lubricity at elevated temperatures. Certain minerals such as bentonite are known to help reduce friction between the drill stem assembly and an open borehole. Materials such as Teflon have been used to reduce sliding contact friction, however these lack durability and strength. Other additives include vegetable oils, asphalt, to graphite, detergents, glass beads, and walnut hulls, but each has its own limitations.
Another prior art method for reducing the friction between pipes is to use aluminum material for the inner string because aluminum is lighter than steel. However, aluminum is expensive and may be difficult to use in drilling operations, it is less abrasion-resistant than steel, and it is not compatible with many fluid types (e.g. fluids with high pH). Alternatively, the industry has developed means to “float” an inner string within an outer string to run casing and liner at high inclinations, but circulation is restricted during this operation and it is not amenable to the hole-making process.
Yet another method for reducing the friction between strings of pipe is to use a hard facing material on the inner string (also referred to herein as hardbanding or hardfacing). U.S. Pat. No. 4,665,996, herein incorporated by reference in its entirety, discloses the use of hardfacing applied to the principal bearing surface of a drill pipe, with an alloy having the composition of: 50-65% cobalt, 25-35% molybdenum, 1-18% chromium, 2-10% silicon and less than 0.1% carbon for reducing the friction between a string and the casing or rock. As a result, the torque needed for the rotary drilling operation, especially directional drilling, is decreased. The disclosed alloy also provides excellent wear resistance on the drill string while reducing the wear on the well casing. Another form of hardbanding is WC-cobalt cermets applied to the drill stem assembly. Other hardbanding materials include TiC, Cr-carbide, and other mixed carbide and nitride systems. A tungsten carbide containing alloy, such as Stellite 6 and Stellite 12 (trademark of Cabot Corporation), has excellent wear resistance as a hardfacing material but may cause excessive abrading of the opposing device. Hardbanding may be applied to portions of the drill stem assembly using weld overlay or thermal spray methods. In a drilling operation, the drill stem assembly, which has a tendency to rest on the well casing, continually abrades the well casing as the drill string rotates.
There are many additional pieces of equipment that have metal-to-metal contact on a drilling rig that are subject to friction, wear, erosion, corrosion, and/or deposits. These devices include but are not limited to the following list: valves, pistons, cylinders, and bearings in pumping equipment; wheels, skid beams, skid pads, skid jacks, and pallets for moving the drilling rig and drilling materials and equipment; topdrive and hoisting equipment; mixers, paddles, compressors, blades, and turbines; and bearings of rotating equipment and bearings of roller cone bits.
Certain operations other than hole-making are often conducted during the drilling process, including logging of the open-hole (or of the cased-hole section) to evaluate formation properties, coring to remove portions of the formation for scientific evaluation, capture of formation fluids at downhole conditions for fluids analyses, placing tools against the wellbore to record acoustic signals, and other operations and methods known to those skilled in the art.
Marine Riser Systems:
In a marine environment, a further complication is that the wellhead tree may be “dry” (located above sea level on the platform) or “wet” (located on the seafloor). In either case, conductor pipes known as “risers” are placed between the surface and seafloor, with drill stem equipment run internal to the riser and with drilling fluid returns in the annular space. Risers may be particularly susceptible to the issues associated with rotating an inner pipe within an outer stationary pipe since the risers are not fixed but may also move due to contact with not only the drill string but also the sea environment. Drag and vortex shedding of a marine riser causes loads and vibrations that are due in part to frictional resistance of the ocean current around the outer surface of the marine riser.
Tubular Goods:
Oil-country tubular goods (OCTG) comprise drill stem equipment, casing, tubing, work strings, coiled tubing, and risers. Common to most OCTG (but not coiled tubing) are threaded connections, which are subject to potential failure resulting from improper thread and/or seal interference, leading to galling in the mating connectors that can inhibit use or reuse of the entire joint of pipe due to a damaged connection. Threads may be shot-peened, cold-rolled, and/or chemically treated (e.g., phosphate, copper plating, etc.) to improve their anti-galling properties, and application of an appropriate pipe thread compound provides benefits to connection usage. However, there are still problems today with thread galling and interference issues, particularly with the more costly OCTG material alloys for extreme service requirements.
Wellhead, Trees, and Valves:
At the top of the casing, the fluids are contained by wellhead equipment, which typically includes multiple valves and blowout preventers (BOP) of various types. Subsurface safety valves are critical pieces of equipment that must function properly in the event of an emergency or upset condition. Subsurface safety valves are installed downhole, usually in the tubing string, and may be closed to prevent flow from the subsurface. Chokes and flowlines connected to the wellhead (particularly joints and elbows) are subject to friction, wear, corrosion, erosion, and deposits. Chokes may be cut out by sand flowback, for example, rendering the measurement of flow rates inaccurate.
Many of these devices rely on seals and very close mechanical tolerances, including both metal-to-metal and elastomeric seals. Many devices (sleeves, pockets, nipples, needles, gates, balls, plugs, crossovers, couplings, packers, stuffing boxes, valve stems, centrifuges, etc.) are subject to friction and mechanical degradation due to corrosion and erosion, and even potential blockage resulting from deposits of scale, asphaltenes, paraffins, and hydrates. Some of these devices may be installed downhole or on the sea floor, and it may be impossible or very costly at best to gain service access for repair or restoration.
Completion Strings and Equipment:
With the drill well cased to prevent hole collapse and uncontrolled fluid flow, the completion operation must be performed to make the well ready for production. This operation involves running equipment into and out of the wellbore to perform certain operations such as cementing, perforating, stimulating, and logging. Two common means of conveyance of completion equipment are wireline and pipe (drill pipe, coiled tubing, or tubing work strings). These operations may include running logging tools to record formation and fluid properties, perforating guns to make holes in the casing to allow hydrocarbon production or fluid injection, temporary or permanent plugs to isolate fluid pressure, packers to facilitate setting pipe to provide a seal between the pipe interior and annular areas, and additional types of equipment needed for cementing, stimulating, and completing a well. Wireline tools and work strings may include packers, straddle packers, and casing patches, in addition to packer setting tools, devices to install valves and instruments in sidepockets, and other types of equipment to perform a downhole operation. The placement of these tools, particularly in extended-reach wells, may be impeded by friction drag. The final completion string left in the hole for production is commonly referred to as the production tubing string.
Formation and Sandface Completions:
In many wells, there is a tendency for sand or formation material to flow into the wellbore. To prevent this from occurring, “sand screens” are placed in the well across the completion interval. This operation may involve deploying a special-purpose large diameter assembly comprising one of several types of sand screen mesh designs over a central “base pipe.” The screen and basepipe are frequently subject to erosion and corrosion and may fail due to sand “cutout.” Also, in high inclination wells, the frictional drag resistance encountered while running screens into the wellbore may be excessive and limit the application of these devices, or the length of the wellbore may be limited by the maximum depth to which screen running operations may be conducted due to friction resistance.
In those wells that require sand control, a sand-like propping material, “proppant,” is pumped in the annular area between the screen and formation to prevent the formation grains from flowing through the screens. This operation is called a “gravel pack” or, if conducted at fracturing conditions, may be called a “frac pack.” In many other formations, often in wellbores without sand screens, fracture stimulation treatments may be conducted in which this same or different type of propping material is injected at fracturing conditions to create large propped fracture wings extending a significant distance away from the wellbore to increase the production or injection rate. Frictional resistance occurs while pumping the treatment as the proppant particles contact each other and the constraining walls. Furthermore, the proppant particles are subject to crushing and generating “fines” that increase the resistance to fluid flow during production. The proppant properties, including the strength, friction coefficient, shape, and roughness of the grain, are important to the successful execution of this treatment and the ultimate increase in well productivity or injectivity.
Artificial Lift Equipment:
When production from a well is initiated, it may flow at satisfactory rates under its own pressure. However, many wells at some point in their life require assistance in lifting fluids out of the wellbore. Many methods are used to lift fluids from a well, including: sucker rod, Corod™, and electric submersible pumps to remove fluids from the well, plunger lifts to displace liquids from a predominantly gas well, and “gas lift” or injection of a gas along the tubing to reduce the density of a liquid column. Alternatively, specialty chemicals may be injected through valves spaced along the tubing to prevent buildup of scale, asphaltene, paraffin, or hydrate deposits.
The production tubing string may include devices to assist fluid flow. Several of these devices may rely on seals and very close mechanical tolerances, including both metal-to-metal and elastomeric seals. Interfaces between parts (sleeves, pockets, plugs, packers, crossovers, couplings, bores, mandrels, etc.) are subject to friction and mechanical degradation due to corrosion and erosion, and even potential blockage or mechanical fit interference resulting from deposits of scale, asphaltenes, paraffins, and hydrates. In particular, gas lift, submersible pumps, and other artificial lift equipment may include valves, seals, rotors, stators, and other devices that may fail to operate properly due to friction, wear, corrosion, erosion, or deposits.
Well Intervention Equipment:
Downhole operations on a wellbore near the reservoir formation interval are often required to gather data or to initiate, restore, or increase production or injection rate. These operations involve running equipment into and out of the wellbore. Two common means of conveyance of completion equipment and tools are wireline and pipe. These operations may include running logging tools to record formation and fluid properties, perforating guns to make holes in the casing to allow hydrocarbon production or fluid injection, temporary permanent plugs to isolate fluid pressure, packers to facilitate a seal between intervals of the completion, and additional types of highly specialized equipment. The operation of running equipment into and out of a well involves sliding contact due to the relative motion of two bodies, thus creating frictional drag resistance.
Therefore, given the expansive nature of these broad requirements for production operations, there is a need for new coating material technologies that protect devices from friction, wear, corrosion, erosion, and deposits resulting from sliding contact between two or more devices and fluid flowstreams that may contain solid particles traveling at high velocities. This need requires novel materials that combine high hardness with a capability for low coefficient of friction (COF) when in contact with an opposing surface. If such coating material can also provide a low energy surface and low friction coefficient against the borehole wall, then this novel material coating may enable ultra-extended reach drilling, reliable and efficient operations in difficult environments, including offshore and deepwater applications, and generate cost reduction, safety, and operational improvements throughout oil and gas well production operations. As envisioned, the use of these coatings on well production devices could have widespread application and provide significant improvements and extensions to well production operations.